This invention is generally related to satellite transponders and, more specifically, to frequency translation routing transponders.
As the traffic demands on commercial communication satellites increase, new techniques must be employed to expand communication capacity. The capacity of a given communication system is determined by the availability of two resources, bandwidth and r.f. power, or signal-to-noise ratio. Bandwidth is normally limited by legal allocation while signal-to-noise ratio is a function of available launch vehicle weight lifting capability and earth station characteristics.
By providing multiple isolated propagation channels, the same allocated frequency spectrum may be reused, thus N-fold increasing the bandwidth resource. The isolation mechanism may be the use of two orthogonal antenna polarizations or spatial pointing isolation provided by spot beams. It is this latter method to which the present invention is directed.
The use of spot beams is one technique for increasing information capacity per bandwidth in a communication satellite system and essentially consists of incorporating multiple antennae on the satellite transponder for transmitting and receiving signals from designated areas on the earth. The beams, known as spot beams, provide spatial diversity and allow multiple communications within the same band. For example, if locations A, B, C and D were "illuminated" (covered by the beam pattern) by spot beams 1, 2, 3 and 4, respectively, a frequency channel occupied by communications from A to B could also be occupied by communications from C to D. This assumes that means are provided in the satellite to interconnect signals from receive beam 1 to transmit beam 2 to interconnect signals from receive beam 3 to transmit beam 4.
For full capacity realization of a multi-spot beam system, there should be means for adjustably interconnecting all or a portion of any receive beam bandwidth to any transmit beam. One such system is described in U.S. Pat. No. 3,810,255, entitled "Frequency Translation Routing Communications Transponder," assigned to the same assignee as the present application. In that system, a plurality of spot beam receiving antennae, e.g., three, each receive a spot beam occupying the same bandwidth, and each received signal is supplied to an N-port filter where it is divided into frequency bands (hereinafter referred to as "band slots" or "slots") which may be individually routed. Each slot is supplied to a mixer where it is mixed with a unique local oscillator signal. The resulting 3 N slots are combined in a power combiner, and the composite signal is fed to a frequency selective power splitter where it is split into three bandwidth segments. Each bandwidth segment is applied to a mixer, down-converted to the desired transmit frequency by a different local oscillator signal, and transmitted via the three transmit beams.
The above-described system, although highly effective, has proven somewhat unsatisfactory in its routing capability. Since the N-port filter at each receive beam antenna divides the received signal into N discrete bandwidths, the ability of the system to allocate te minimum possible bandwidth to each transmit antenna is limited to combinations of those bandwidths, thereby resulting in overall system inefficiency. By way of example, let us assume that a 5-port filter is provided at each receive beam antenna for separating the received signal into five equal discrete bandwidths. If the received signal occupies a 0.5 GHz bandwidth, each of the five output signals from the filter will occupy a 0.1 GHz bandwidth. Let us further assume that the system includes three receive beam antennae A, B and C and three transmit beam antennae AA, BB and CC. If only three channels, one at each of antennae A, B and C, are to be transmitted by antenna AA, it will be necessary to allocate to transmit antenna AA one discrete 0.1 GHz bandwidth from each antenna. This results in a routing inefficiency since a 0.3 GHz bandwidth is much greater than that required for three channels. It would be possible to increase the efficiency of the system by decreasing the size of the discrete bandwidths--i.e., providing a 10-port filter rather than a 5-port filter at each receive beam antenna--but this would result in a substantial increase in the weight of the transponder which is highly undesirable in the satellite environment. Moreover, even a 5-port filter is undesirably large for a satellite transponder system in view of its limited efficiency.